This invention relates generally to digital communication systems, and more particularly to a spectrum spreading technique for use in multi-node digital communication systems such as digital networks and digital radios.
Spectrum spreading techniques for use in digital communication networks have been described in many books and papers. A classic publication in this field is Spread Spectrum Communications by M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levift, Computer Science Press, 11 Taft Court, Rockville, Md. 20850, 1985. Particular kinds of spectrum spreading techniques that have been implemented in digital communication networks in the prior art include xe2x80x9cdirect-sequence spreadingxe2x80x9d, xe2x80x9cfrequency hoppingxe2x80x9d, xe2x80x9ctime hoppingxe2x80x9d, and various hybrid methods that involve combinations of the aforementioned techniques.
Multi-node spread-spectrum communication networks developed in the prior art were generally characterized as code-division multiple-access (CDMA) networks, which utilized xe2x80x9ccode-division multiplexingxe2x80x9d (i.e., a technique in which signals generated by different spreading-code sequences simultaneously occupy the same frequency band). Code-division multiplexing requires that the simultaneously used spreading codes be substantially xe2x80x9cmutually orthogonalxe2x80x9d, so that a receiver with a filter matched to one of the spreading codes rejects signals that have been spread by any of the other spreading codes.
In a typical multi-node spread-spectrum communication network using either a conventional direct-sequence spectrum spreading technique, or a hybrid technique involving,e.g., direct-sequence and frequency-hopped spectrum spreading, only a single spreading code is employed. At regular intervals, the polarity of the spreading code is either inverted (i.e., each 0 is changed to 1, and each 1 is changed to 0) or left unchanged, depending on whether the next bit of information to be transmitted is a 1 or a 0. The resulting signal is an xe2x80x9cinformation-bearingxe2x80x9d sequence, which ordinarily would be transmitted using some type of phase-shift keyed (PSK) modulationxe2x80x94usually, binary phase-shift keyed (BPSK) modulation or quaternary phase-shift keyed (QPSK) modulation.
A publication entitled Spread Spectrum Techniques Handbook, Second Edition, March 1979, which was prepared for the National Security Agency by Radian Corporation of Austin, Tex., describes a number of spread-spectrum techniques that had been proposed in the prior art. Of particular interest is a direct-sequence technique described on page 2-21 et seq. of the Spread Spectrum Techniques Handbook, which involved transmitting one bit of information (either a 0 or a 1) by switching between two independent signals that are generated by different spreading codes. Ideally, the spreading codes of the two independent signals should be xe2x80x9calmost orthogonalxe2x80x9d with respect to each other, so that cross-correlation between the two sequences is very small. In practice, in such early spread-spectrum communication systems, the two independent signals were maximal-length linear recursive sequences (MLLRSs), often called xe2x80x9cM-sequencesxe2x80x9d, whose cross-correlations at all possible off-sets had been computed and found to be acceptably low. However, this technique of switching between two independent signals did not achieve widespread acceptance, mainly because it required approximately twice the electronic circuitry of a polarity-inversion technique without providing any better performance.
Two recent papers, viz., xe2x80x9cSpread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receiversxe2x80x9d by P. K. Enge and D. V. Sarwate, (IEEE Transactions on Communications, Vol. COM-35, No. 12, December 1987, pp. 1309-1319), and xe2x80x9cSpread-Spectrum Multiple-Access Performance of Orthogonal Codes for Indoor Radio Communicationsxe2x80x9d by K. Pahlavan and M. Chase, (IEEE Transactions on Communications, Vol. 38, No. 5, May 1990, pp. 574-577), discuss multi-node spread-spectrum communication networks in which multiple orthogonal sequences within a relatively narrow bandwidth are assigned to each node, whereby a corresponding multiplicity of information bits can be simultaneously transmitted and/or received by each nodexe2x80x94thereby providing a correspondingly higher data rate. A specified segment of each sequence available to a node of the network is designated as a xe2x80x9csymbolxe2x80x9d. In the case of a repetitive sequence, a symbol could be a complete period of the sequence. The time interval during which a node transmits or receives such a symbol is called a xe2x80x9csymbol intervalxe2x80x9d. In a multi-node spread-spectrum network employing multiple orthogonal sequences, all the nodes can simultaneously transmit and/or receive information-bearing symbols derived from some or all of the sequences available to the nodes.
The emphasis in the aforementioned Enge et al. and Pahlavan et al. papers is on network performance, especially in certain kinds of signal environments. Neither paper recommends or suggests using any particular set of mutually orthogonal spreading codes for generating multiple orthogonal sequences; and neither paper discloses how to derive or generate suitable mutually orthogonal spreading codes. However, methods of generating families of sequences that are pairwise xe2x80x9calmost orthogonalxe2x80x9d by using two-register sequence generators have been known for some time.
In a paper entitled xe2x80x9cOptimal Binary Sequences for Spread-Spectrum Multiplexingxe2x80x9d by R. Gold, (IEEE Transactions on Information Theory, Vol. IT-13, October 1967, pp.119-121), so-called xe2x80x9cGold codesxe2x80x9d were proposed for use as spreading codes in multi-node direct-sequence spread-spectrum communication networks of the CDMA type. A Gold code is a linear recursive sequence that is generated by a product f1f2, where f1 and f2 comprise the members of a so-called xe2x80x9cpreferred pairxe2x80x9d of primitive polynomials of the same degree n over a field GF(2). A primitive polynomial of degree n is defined as a polynomial that generates a maximal-length linear recursive sequence (MLLRS), which has a period of (2nxe2x88x921). The required relationship between f1 and f2 that makes them a preferred pair is described in the aforementioned paper by R. Gold.
A Gold code is a particular kind of xe2x80x9ccomposite codexe2x80x9d. Other kinds of composite codes include xe2x80x9csymmetric codesxe2x80x9d and xe2x80x9cKasami codesxe2x80x9d. A symmetric code is similar to a Gold code in being generated by a product f1f2 of a pair of primitive polynomials, except that for a symmetric code the polynomial f2 is the xe2x80x9creversexe2x80x9d of primitive polynomial f1, i.e., f2(x)=xnf1(1/x), where n=deg f1=deg f2. The correlation properties of Gold codes and symmetric codes are discussed in a paper entitled xe2x80x9cCross Correlation Properties of Pseudorandom and Related Sequencesxe2x80x9d by D. V. Sarwate and M. B. Pursley (Proceedings of the IEEE, Vol. 68, p.5 May 1980, pp. 593-619). Kasami codes differ from Gold codes in that for Kasami codes, the polynomials f1 and f2 are not of the same degree. Kasami codes are also discussed in the aforementioned paper by M. B. Pursley and D. V. Sarwate. The concept of a xe2x80x9ccomposite codexe2x80x9d can be broadened to include sequences obtained from a two-register sequence generator, where the sequences generated in the two registers can be quite general.
Predominant among the reasons that have militated against using direct-sequence spreading codes for multi-node spread-spectrum communication networks of the prior art is the so-called xe2x80x9cnear-farxe2x80x9d problem. If the nodes of a multi-node spread-spectrum communication network are widely distributed so that power levels for different nodes can differ markedly at a given receiver in the network, then at the given receiver the correlations of a reference sequence with a sequence that is transmitted by a nearby node are apt to be stronger than correlations of the reference sequence with a version of the reference sequence that has been transmitted from a greater distance. Adverse effects of the xe2x80x9cnear-farxe2x80x9d problem can include periodic strong correlations in information-bit errors, and false synchronization. To avoid such adverse effects, frequency hopping has been preferred in the prior art for multi-node spread-spectrum communication networksxe2x80x94especially for tactical networks where the nodes are widely distributed. Until recently, most of the research funding and efforts in connection with multi-node spread-spectrum communication networks have been directed toward tactical networks, thereby virtually precluding significant research on direct-sequence spread-spectrum communication networks.
Hybrid frequency-hopped and direct-sequence spread-spectrum communication networks have been proposed for tactical applications. However, the frequency diversity provided by xe2x80x9choppingxe2x80x9d of the carrier readily enables rejection of unintended signals, thereby making the choice of a particular spreading-code sequence relatively unimportant. Consequently, there has been substantially no research in the prior art on the use of Gold codes and other composite codes for hybrid frequency-hopped and direct-sequence spread-spectrum communication networks.
Direct-sequence spread-spectrum communication networks have received recent attention in connection with the development of wireless local area networks (LANs), personal communications networks (PCNs), and cellular telephone networks utilizing communications satellites. The xe2x80x9cnear-farxe2x80x9d problem is ordinarily not an issue for LANs and PCNs, because the nodes in such networks are generally distributed at distances that are not very far from each other. For cellular telephones, the xe2x80x9cnear-farxe2x80x9d problem is not an issue in satellite applications, because all transmitters in the xe2x80x9cspot beamxe2x80x9d from a satellite are roughly at the same distance from the satellite.
Several wireless LANs are described in an article entitled xe2x80x9cSpread Spectrum Goes Commercialxe2x80x9d by D. L Schilling, R. L Pickholtz and L. B. Milstein, IEEE Spectrum, Vol.27 No.6 August 1990, pp. 40-45. For indoor spread-spectrum communication networks (e.g., wireless LANs), spectrum spreading has commonly been employed in xe2x80x9cstar networkxe2x80x9d configurations. In a star network, the nodes are normally synchronized with a master controller, so that each node of the network can use a different offset of the same spreading-code sequence. False synchronization is not ordinarily encountered with star networks. In circumstances in which two or more star networks, each utilizing a different spreading-code sequence, operate in close proximity to each other, composite codes could be used to advantage to prevent interference between neighboring star networks. However, in the prior art, reliance has usually been placed upon the distance between the individual star networks, and upon signal-attenuating structures (e.g., walls) separating the individual star networks, as well as upon cross-correlation properties that are expected of random uncorrelated spreading-code sequences, to enable one star network to reject signals from another star network in its vicinity. Consequently, composite codes have generally not been used in star networks.
In PCNs, the use of composite codes as spreading-code sequences has not yet received much attention, because factors such as size, weight and power considerations have generally favored simplicity over performance. Techniques involving satellite-based CDMA cellular radio networks have emerged from developments in wireless LANs, but have generally been concerned with coding and systems engineering rather than with spreading-code sequence generation.
To date, direct-sequence spectrum spreading techniques have been used primarily in applications requiring high multipath immunity, good time resolution, robustness, privacy and low probability of detection, and for which in-band interference and the xe2x80x9cnear/farxe2x80x9d problem are manageable. Such applications have included satellite communications, star networks in office environments, mobile radio, and positioning and navigation applications. The use of composite codes (e.g., Gold codes or symmetric codes) for spectrum spreading in such applications has not heretofore been deemed appropriate, because composite codes would require significantly greater hardware complexity to implement than MLLRSs without seeming to provide sufficient compensating advantages over MLLRSs in terms of processing gain, the number of nodes that can be accommodated, the rate of data transmission, or robustness.
It is a general object of the present invention to provide a spread-spectrum technique for use in a multi-node digital communication network, whereby a unique set of spreading-code sequences is assigned to each node of the network for transmitting digital signals.
It is a particular object of the present invention to provide a method for generating a family of nearly orthogonal spreading-code sequences, and for assigning a unique set of spreading-code sequences from the family of sequences so generated to each node of a multi-node digital communication network.
It is also a particular object of the present invention to provide methods for selecting a set of one or more spreading-code sequences that can be used during a specified period of time (i.e., a so-called xe2x80x9csymbol intervalxe2x80x9d) to convey multiple bits of information, if the selected sequence or sequences of the set are modulated and transmitted simultaneously.
It is likewise a particular object of the present invention to provide logic circuit designs for hardware implementation of methods for generating a family of spreading-code sequences for assignment to the nodes of a multi-node digital communication network.
It is a further object of the present invention to provide methods for simultaneously modulating a set of carriers of the same frequency but of different phases in order to enable multiple bits of information to be transmitted on each carrier of the set.
It is another object of the present invention to provide a spread-spectrum technique for use in a multi-node digital communication network, which can readily incorporate standard error-control coding (whose parameters are matched to the particular application) into the transmission and reception of digital signals propagated by the network.
It is also an object of the present invention to provide a technique whereby conventional equipment designed for generating arbitrary spreading-code sequences can be adapted to the task of generating a family of spreading-code sequences for use in a multi-node digital communication network.
It is a further object of the present invention to provide a technique whereby direct-sequence spectrum spreading, or a hybrid combination of direct-sequence and frequency-hopped spectrum spreading, can be utilized in conjunction with code diversity or xe2x80x9ccode hoppingxe2x80x9d in a spread-spectrum digital communication network designed to have a low probability of intercept (LPI).
It is also an object of the present invention to provide symbol detection methods, which enable a receiver at any given node in a multi-node spread-spectrum digital communication network to determine the most likely spreading-code sequence or sequences transmitted by another node of the network attempting to communicate with the given node.